It has been considered from the viewpoint of global environment protection to use a fuel cell as a power supply for a motor operable in place of an internal engine for automobiles, to drive an automobile by the motor. The fuel cell does not need the use of a fossil fuel accompanied by the problem of resource depletion, and can be free from emissions such as exhaust gases. The fuel cell further has eminent advantages, such that it is fairly noise-free, and affords the efficiency of energy collection to be higher than other energy machinery.
Fuel cells are categorized in accordance with the kind of electrolyte in use, into a polymer electrolyte type, a phosphate type, a molten carbonate type, a solid oxide type, etc. As one of them, the polymer electrolyte type (PEFC: Polymer Electrolyte Fuel Cell) is a fuel cell that employs as its electrolyte a membrane of electrolyte of a polymer having proton exchange groups in the molecule, making use of a function the polymer electrolyte membrane exhibits as a proton-conductive electrolyte with a saturated content of water. This polymer electrolyte fuel cell works at relatively low temperatures, and with a high efficiency of power generation. Moreover, the polymer electrolyte fuel cell is allowed to be small in size and light in weight, together with other associated equipment, and is expected to have a variety of applications including mounting to electric vehicles.
The polymer electrolyte fuel cell includes a fuel cell stack. The fuel cell stack is integrally configured as a lamination of a plurality of unit cells each working as a fundamental unit for power generation by electrochemical reactions, while the lamination is sandwiched between end flanges put on both ends thereof, and held tightened by tie bolts. The unit cells are each configured with a solid polymer electrolyte membrane, and a combination of an anode (hydrogen electrode) and a cathode (oxygen electrode) joined to both sides thereof.
FIG. 22 is a schematic sectional view of the configuration of a unit cell constituting a fuel cell stack. As shown in FIG. 22, the unit cell 200 includes a membrane electrode assembly, which is made up by a solid polymer electrolyte membrane 201, and a combination of an oxygen electrode 202 and a hydrogen electrode 203 integrally joined to both sides thereof. The oxygen electrode 202 and the hydrogen electrode 203 each respectively have a two-layered structure composed of a reaction film 204 and a gas diffusion layer (GDL) 205, and the reaction film 204 contacts on the solid polymer electrolyte membrane 201. On both sides of the combination of oxygen electrode 202 and hydrogen electrode 203, an oxygen electrode side separator 206 and a hydrogen electrode side separator 207 are arranged for lamination, respectively. By the oxygen electrode side separator 206 and the hydrogen electrode side separator 207, there are defined oxygen gas channels, hydrogen gas channels, and cooling water channels.
For manufacture of the unit cell 200 configured as described, the oxygen electrode 202 and the hydrogen electrode 203 are disposed on both sides of the solid polymer electrolyte membrane 201, and integrally joined thereto, typically by a hot pressing method, to form the membrane electrode assembly, and then, the separators 206 and 207 are disposed on both sides of the membrane electrode assembly. The unit cell 200 constitutes a fuel cell, where a gaseous mixture of hydrogen, carbon dioxide, nitrogen, and water vapor is supplied at the side of hydrogen electrode 203, and air with water vapor, at the side of oxygen electrode 202, whereby electrochemical reactions are caused principally at contact surfaces between solid polymer electrolyte membrane 201 and reaction films 204. More specific reactions will be described below.
In the above-noted configuration of unit cell 200, with oxygen gases and hydrogen gases distributed to oxygen gas channels and hydrogen gas channels, respectively, oxygen gases and hydrogen gases are supplied through gas diffusion layers 205 toward the reaction films 204, causing the following reactions in the reaction films 204.Hydrogen electrode side: H2→2H++2e−  formula (1)Oxygen electrode side:(½)O2+2H++2e−→H2O  formula (2)At the side of hydrogen electrode 203, with hydrogen gas supplied, the reaction of formula (1) proceeds, producing H+ and e−. H+ is hydrated, which moves in the solid polymer electrolyte membrane 201, flowing toward the oxygen electrode 202, while e− is conducted through a load 208, flowing from the hydrogen electrode 203 to the oxygen electrode 202. At the side of oxygen electrode 202, with H+ and e− and oxygen gas supplied, the reaction of formula (2) proceeds, generating electric power.
For fuel cells, separators bear a function of electrical connection between unit cells, as described, and need a good conductivity of electricity, and low contact resistances to component materials of gas diffusion layers and the like.
Moreover, the electrolyte membrane of solid polymer type, made of a polymer with multiple sulfonate groups, is humidified to employ sulfonate groups as proton exchange groups, so as to be proton-conductive. For the electrolyte membrane of solid polymer type, which is strongly acidic, the fuel cell separators are required to be corrosive-resistant against sulfate acidities around pH2 to pH3.
Still more, for fuel cells, gases to be supplied have as hot temperatures as within 80 to 90° C., in addition to that there is provided not simply the hydrogen electrode in which H+ is produced, but also the oxygen electrode, where oxygen as well as air or the like passes, constituting an oxidizing environment in which the electric potential to be born ranges from a natural potential to a maximum of about 1 V vs. SHE relative to a standard hydrogen electrode potential. Hence, as well as the oxygen electrode and the hydrogen electrode, the fuel cell separators are required to have a corrosion resistance endurable under a strongly acidic atmosphere.
It is noted that the corrosion resistance now required means a durability that permits the fuel cell separator to have a maintained performance of electric conduction even under a strongly acidic environment. In other words, the corrosion resistance should be measured in an environment in which cations are transferred into humidifying water or production water due to the reaction of formula (2), and are bonded with sulfonate groups that inherently should have made ways for protons, occupying those sulfonate groups, causing power generating characteristics of electrolyte membrane to deteriorate.
Further, although fuel cells have a theoretical voltage, which is 1.23 V per unit cell, the voltage that can be actually taken out is dropped due to reaction polarization, gas diffusion polarization, and resistance polarization. As the current to be taken out gets greater, the voltage drop also increases. Furthermore, in applications to automobiles, where increasing power density per unit volume or unit weight is wanted, the service may well be provided at a greater current density than for stationary use, e.g., at a current density of 1 A/cm2. In this respect, it is considered that for the current density of 1 A/cm2, if the contact resistance between separator and electrode is kept within a range of 40 mΩ·cm2 or less, the efficiency reduction due to contact resistance is controllable.
To this point, for separators for fuel cells, attempts have been made to employ an electrically well conductive and excellently corrosion-resistive stainless steel or titanium material such as a pure titanium material for industrial use. The stainless steel has a dense passive film formed on the surface, with oxides or hydroxides, or hydrates of them or the like, containing chromium as a principal metallic element. Likewise, the titanium material has a dense passive film formed on the surface, with titanium oxides or titanium hydroxides, or hydrates of them or the like. The stainless steel as well as the titanium material is thus well anti-corrosive.
However, the above-noted passive films have contact resistances to a carbon paper employed typically as a gas diffusion layer. Fuel cells have an excessive voltage due to a resistance polarization therein, while for stationary applications affording a waste heat collection such as by co-generation, the heat efficiency can be enhanced as a total. But, for applications to automobiles, where the heat loss due to contact resistance has to be simply wasted outside, through cooling water, from a radiator, the efficiency of power generation decreases, as the contact resistance increases. Further, the decrease in efficiency of power generation is equivalent to an increase in heat dissipation, which leads to the need for a greater cooling system to be installed. Therefore, the increase of contact resistance has come up as an important issue to be solved.
In this respect, there is proposed a separator for fuel cells in Japanese Patent Application Laying-Open Publication No. 10-228914 (refer to page 2, and FIG. 2), in which a stainless steel is press-formed, and thereafter, a gold film is plated directly on surface regions to be brought into contact with an electrode. Further, there is proposed a separator for fuel cells in Japanese Patent Application Laying-Open Publication No. 2001-6713 (refer to page 2) in which a stainless steel is molded and machined in the form of a separator for fuel cells, and thereafter, passive films of surface regions that will have contact resistances when brought into contact with an electrode are removed, and a precious metal or a precious metal alloy is attached.